This invention relates in general to filters for use in blood vessels. In particular, this invention relates to an improved structure for an intravascular filter that includes at least two biodegradable filter sections that degrade at different rates.
An embolism is a detached intravascular mass that is carried by circulation through blood vessels in a living body and that is capable of clogging a vessel that is too small to let it pass therethrough. When this happens, the flow of blood through the vessel is undesirably stopped by the embolus. In many instances, an embolus is a small piece of a blood clot that breaks off from a site of trauma or other injury and travels to another site in the living body.
One type of embolism is a pulmonary embolism, which is a blockage in a lung artery by an embolus. A pulmonary embolism can cause damage to lung tissue from the obstruction of blood flow. Further, low oxygen blood levels may be caused by the pulmonary embolism, which may cause damage to other organs due to oxygen deprivation. If the pulmonary embolism is severe enough, it may even result in death. A pulmonary embolism is one of the more common causes of death in people who remain in bed for a long time. In most cases, a pulmonary embolism is a complication of a condition called deep vein thrombosis (DVT). In DVT, blood clots form in the deep veins of the body, most often in the legs. These clots can break free and travel to the lung, causing the pulmonary embolism.
Intravascular filters are well known devices that are used to restrain emboli from traveling throughout a living body. Typically, these intravascular filters are inserted within a blood vessel in order to restrain the emboli from traveling therethrough until the normal processes of the body are capable of partially or fully dissolving them. The inferior vena cava (IVC) is a large vein that returns blood from the lower half of the body to the heart. In many instances, the IVC is a desirable location for placement of an intravascular filter. Intravascular filters are generally inserted within the IVC in patients at risk for DVT and who cannot undergo standard anticoagulation therapy (or in patients where anticoagulation therapy is not effective). As the delivery system profile of these intravascular filters has decreased and the ease of inserting same improved, more of such filters are being utilized. This is especially true with patients who have multiple trauma or major orthopedic procedures.
Until recently, most intravascular filters were permanent type filters, meaning that even though the need for protection from pulmonary emboli was relatively short (measured in days to months), the intravascular filter was simply left in the patient after the need for protection had passed. This was because the intravascular filter could only be removed with another relatively major surgical operation. Thus, the risk of removing the filter was greater than the risk of leaving the filter in place. Consequently, rather than subjecting the patient to a second operation, the intravascular filter is often simply left in the patient after the need for protection had passed.
More recently, however, recoverable intravascular filters have been developed. Such recoverable intravascular filters can be removed when the need for protection ends using a less invasive procedure than previously known. Furthermore, it is known to provide intravascular filters that include some bio-absorbable components, wherein at least a portion of these filters will be broken down and absorbed by the body over time. This allows for protection from a pulmonary embolism for a period of time without requiring recovery of at least a portion of the filter. Such a filter is described in U.S. Pat. No. 5,375,612 to Cottenceau et al. However, risks are still presented by the use of such procedures and devices. Thus, it would be desirable to provide an improved structure for a intravascular filter that addresses these issues.